Printed wiring boards generally have such a structure as shown in FIGS. 1 and 2 of the accompanying drawings. FIG. 1 is a plan view showing a wiring surface 2 of a printed wiring board 1, and FIG. 2 is a sectional view of the printed wiring board shown in FIG. 1, taken along the line II--II. In FIGS. 1 and 2, reference numerals 4 and 5 denote conductor pieces constituting a wiring pattern formed of a conductor foil such as a copper foil, 6 plated through holes for electrically connecting the wiring surface 2 with the back wiring surface 3, 7 an insulating substrate, and 13 and 14 conductor pieces constituting an internal layer pattern used respectively as ground and power layers and formed of a conductor foil such as a copper foil. The wiring pattern shown in FIG. 1 is formed by transferring a pattern which is indicated by solid lines (excepting heavy lines) and dotted lines in FIG. 1, on a copper foil in the form of an etching resist pattern by the use of a photographic plate (or a mask), and by etching the copper foil. When dust adheres to the photographic plate or a flaw is produced therein, there appear on the wiring pattern 2 a fine undesired pattern 8, a fine partial lack of pattern 9, and a fine projection 10. Further, the thickening 11 or thinning 12 of pattern is produced according as the etching operation is performed insufficiently or excessively. Such defects as above give rise to various problems. That is, the fine partial lack of pattern 9 and the thinning of pattern 12 increase the electric resistance of the pattern, decrease the current capacity of the pattern, and give rise to disconnection when the printed wiring board is subjected to slight rubbing. The fine undesired pattern 8, the fine projection 10 and the thickening 11 of pattern give rise to a short circuit, or a solder bridge in soldering process. As a result, a correct (or desired) wiring cannot be formed on the printed wiring board. Specifically, in a recent high density mounting which employs, for example, a pattern width of 0.1 mm, it is required to detect the above-mentioned defects without overlooking them. However, since such detection cannot be made by visual inspection, the apparatus shown in FIG. 3 is employed in which an optical image is formed for each of the printed wiring boards 1 to be inspected and another printed wiring board 1' for comparison and collation to be compared and collated with each other. The conventional method employing the above-mentioned apparatus will be explained below. In FIG. 3 which shows a conventional apparatus for inspection of printed wiring boards, the same structures are arranged on the right and left sides with the exception of electrical-signal collating device 27, and the part on the right side corresponding to each of the parts on the left side is given the same reference numeral with prime. Explanation will not be made on the function and operation of each part on the right side, because the explanation thereof is given by replacing a reference numeral by the same reference numeral with prime in the following explanation made on the function and operation of each part on the left side. Above the wiring surface 2 of the printed wiring board 1 are disposed a half reflecting mirror 23 which reflects the horizontal light from a light source 21 to produce the light incident upon the wiring surface 2, and through which the reflected light from the wiring surface 2 travels upward, a refractor 24 which converges the light having passed through the mirror 23 to form an optical image, and a photodiode array 25 which is placed in an image forming plane and converts a pattern of light and darkness in the formed image into a multiplicity of electrical signals 26. Further, a collating device 27 is provided in which both the electrical signals 26 delivered from the left photodiode array 25 and the electrical signals 26' delivered from a right-hand side photodiode array 25' are recognized as wiring patterns, and are compared and collated with each other to point out the presence or absence of defects or positions where the defects exist. The positioning of each of the printed wiring boards 1 and 1' is made by positioning means (not shown). Now, explanation will be made on a case where, for example, a printed wiring board shown in FIG. 4a is inspected. Referring to FIG. 3, the light emitted from the light source 21 is passed through a refractor 22 to form parallel rays, directed downward by the half reflecting mirror 23, and then incident upon various portions on the wiring surface 2 of the printed wiring board 1 as light rays 31, 32 and 33 shown in FIG. 4a. The light ray 31 is reflected back as the reflected light ray 41 of a low intensity level due to a low reflectivity of the insulating substrate 7, the light ray 32 is reflected back as the reflected light ray 42 of a high intensity level due to a high reflectivity of the wiring pattern 5 made of a metal such as copper, and the light ray 33 does not give rise to reflected light because it goes past to the back wiring surface 3 through the plated through hole 6 or a perforation. The reflected light rays 41 and 42 which are directed upward, are incident upon the surface of the photodiode array 25 through the half reflecting mirror 23 and the refractor 24 to form an optical image. The photodiode array 25 includes a multiplicity of fine photodiodes (or light receiving elements) which are arranged on a straight line. For example, 256 photodiodes are arranged on a straight line as long as 5 mm. FIG. 4b is a waveform chart for showing electrical signals generated by the individual photodiodes when the light rays 41 and 42 form the optical image. In FIG. 4b, the abscissa designates the location of each photodiode of the photodiode array 25, and the ordinate the level of each of the electrical signals. Further, reference symbol I.sub.1 denotes the level of electrical signals corresponding to the position of the plated through hole 6, which is low due to the absence of reflected light, I.sub.2 the level of electrical signals into which the reflected light from the insulating substrate 7 is converted, which level is low but higher than I.sub.1, and I.sub.3 the level of electrical signals into which the reflected light from the wiring pattern 5 is converted, which level is high. In order to facilitate the comparison of these levels, it is necessary for these three levels to be converted into two kinds of levels (light and dark levels) or binary levels. For this reason, there is provided a binary coder 28 which is formed of, for example, a voltage comparator, and which translates an electrical signal having a signal level higher than a level I.sub.s (shown in FIG. 4b) to the light level and an electrical signal of a signal level lower than the level I.sub.s to the dark level. Thus, the electrical signals based upon the wiring pattern 5 are translated to the light level (or "1" level of binary code), and those based upon the insulating substrate 7 and plated through hole 6 are translated to the dark level (or "0" level of binary code). That is, the electrical signals delivered from the photodiode array 25 are converted into binary signals. The binary signals thus obtained form linear information (that is, such linear information as viewing the pattern of FIG. 5a across the line V.sub.b --V.sub.b), since the light receiving surface of the photodiode array 25 has a form of a line. Accordingly, by storing these binary signals in a memory 29 while displacing the printed wiring board 1 in parallel in the plane containing the wiring surface 2, the plane information can be obtained. Then, the plane information on the wiring surface 2 and that on the wiring surface 2', both of which have been stored in the memory 29, are collated with each other at a pattern comparator 30 to indicate those parts which correspond to but are incongruous with each other, as a defect.
According to the above-mentioned inspecting apparatus, since the light rays 31 and 32 are incident upon the wiring surface 2 from above as shown in FIG. 4a, the reflected light rays 41 and 42 are directed upwardly as far as the wiring surface is flat, and positively converged by the refractor 24 to form an optical image, a light and dark pattern of which is converted to an electrical signal 26 by each photodiode in the photodiode array 25. The plated through holes 6 included in the wiring pattern usually expand at its opening portion on the wiring surface 2 so that the wall defining the hole has a corner 15 rounded at the opening portion, as shown in FIG. 2. As a result, the light incident upon the curved surface of the corner 15 cannot be reflected upwardly pursuant to the law of light ray reflection. Accordingly, the reflected light is not converged by the refractor 24, failing to take part in the production of pattern information and a diameter larger than that of an actual plated through hole is recognized. Therefore, the conventional inspecting apparatus in which light is incident upon the wiring surface from above is disadvantageously invalid for inspecting pattern information concerning the corner of the wall of the plated through hole, which corner is simply referred to as a plated through hole corner hereinafter.
To detail a defect at a plated through hole corner, reference is now made to FIGS. 5a and 5b. FIG. 5a is a plan view showing a wiring surface 2 of a printed wiring board 1, and FIG. 5b is a sectional view of the printed wiring board shown in FIG. 5a, taken along line V.sub.b--V.sub.b. In fabricating a printed wiring board by using an etching resist in the form of a dry film, a defect 16 at a plated through hole corner as shown in FIG. 5a often takes place when a dry film tent applied over the plated through hole for protecting the same is damaged during the etching treatment. The presence of the defect tends to cause troubles in soldering parts to the printed wiring board and breakage of the plated through hole corner.